Manchester welcomed back Paul Pogba to the starting line-up, whilst West Ham welcomed the return of front-men Marko Arnautovic.
The Hammers wasted no time getting on the act after Pablo Zabaleta whipped in a low cross for Felipe Anderson to finish off exquisitely in the central attacking area on six minutes.
After struggling to get going, the embattled Red Devils found their first effort on target as Romelu Lukaku hit the post with a header at the midway point.
For West Ham, it looked all too comfortable heading towards halftime with their counter-attacking approach looking threatening much the time.
On the other hand, the struggle for the more fancied Manchester side got worse from time to time and despite the large share of possession, they were unable to hurt the hosts in the treacherous arrears.
The Londoners looked to finish the half strongly, and so did after Andriy Yarmolenko brought down a ball from a corner at the far post to curl in the left corner off a deflection from Victor Lindelof.
In the second half, the home side applied the same approach that worked wonderfully well for them in the opening period.
On 50 minutes, Arnautovic found huge space down the left side off a counter but blasted dismally off target.
As the slow-enduing approach not brought any fruits, the visitors made a change. Marcus Rashford replaced Lindelof five minutes later for an all-out offensive approach.
In the 63rd minute, Lukasz Fabianski was called to his first save with Marouane Fellaini’s header at the far post off a cross out wide from Ashley Young.
The impact of Rashford proved fruitful after he sneaked ahead of the defence to flick home in the bottom corner 20 minutes from time.
The joy was short-lived though as Arnautovic beat the offside trap for a low bottom tuck finish to reimburse their two-goal lead.
Mourinho’s men looked dead and buried from then on, but a well-driven effort from Scott McTominay asked serious questions of Fabianski who came up with the goods in the end.
In the closing stages, substitute Michail Antonio attempted to add a fourth after getting down the byline, but shot straight at a starring David De Gea, as the Hammers earned a big scalp.